1. Field of the Invention
The present invention relates to an in-focus detecting apparatus suitable for use in an apparatus to detect the in-focus condition by use of a video signal obtained from image pickup means.
2. Description of the Related Background Art
Hitherto, there has been proposed an automatic focusing system by what is called a mountain climbing servo system (Japanese Laid-Open Patent Gazette No. 188966/1983 ) in which a video signal which was output from an image pickup device or the like is supplied to a plurality of band pass filters (BPF) having different pass bands. The fineness of the photographing screen is detected from the video signal by the output of each BPF, and the position of the focusing lens is adjusted so as to obtain the maximum fineness.
A block diagram of the conventional automatic focus matching apparatus is shown in FIG. 1. In this apparatus, the optical information of an object to be photographed which is received through a lens 1 is converted into an electric signal by a camera tube 2 and amplified by a preamplifier 3 and thereafter, it is converted into a television signal by a process circuit 4. A BPF 13 has a low center frequency and its band lies within a range from 200 kHz to 1 MHz, so that although the detecting range is wide, the detecting sensitivity is low. On the other hand, in the case of a BPF 14, the center frequency is high and the band lies within a range from 15 MHz to 25 MHz. Therefore, although the detecting range is narrow, the detecting sensitivity is higher than that of the BPF 13 and is excellent. An operation processing circuit 15 has a function of what is called a change-over switch for allowing an output of the BPF 13 to pass when the focus adjusting position of the lens 1 is largely deviated from the focal point and for allowing an output of the BPF 14 to pass at a position near the focal point. The decision of this switching is performed on the basis of whether the output of the BPF 14 is at a predetermined threshold value or more or not. In the focus matching apparatus, the lens 1 is finely vibrated in the direction of an optical axis at a reference frequency. A reference frequency component detecting circuit 8 fetches the relevant reference frequency component from an output of the operation processing circuit 15 and the synchronous detection is performed by a synchronous detection circuit 9. An output of the synchronous detection circuit 9 is supplied to a motor drive circuit 11, thereby controlling the lens 1 so as to move the lens 1 to the focal point.
FIG. 2 shows an example of the output of the BPF which is obtained by such a system. Since the BPF output voltage (hereinafter, referred to as a focal voltage) corresponds to the fineness of the photographed video image, the focal voltage becomes maximum in the situation where the image forming surface of the photographing lens coincides with the image pickup surface of the camera tube, namely, in the infocus state. As the image forming surface of the photographing lens is deviated from the image pickup surface of the camera tube, the focal voltage gradually decreases. Therefore, by adjusting the position of the photographing lens so as to obtain the maximum focal voltage, the focal point can be automatically adjusted. Since the curve of the focal voltage generally largely changes in accordance with an object to be photographed or focus matching state, the BPF having a large output level must be selected each stage of the focus adjustment. In general, the output of the BPF.sub.1 for the middle frequency band is used in the largely faded state with the low fineness, and the output of the BPF.sub.2 for super high frequency band is used at a position near the focal point in order to raise the stop accuracy of the lens.
However, with the foregoing constitution, since two BPF outputs cannot be simultaneously processed by the operation processing circuit 15, the decision in selection of the BPF must be made on the basis of a proper threshold value. As is well known, since the output level of the BPF largely differs in dependence on an object to be photographed, it is very difficult to set such a threshold value to select the BPF, causing a focus malfunction.
On the other hand, the focal voltage itself largely depends on the brightness of an object to be photographed. In particular, under the illumination by a fluorescent lamp, the focal voltage of each field causes a level fluctuation at a certain period. For example, in the case where the field frequency is 60 Hz and the power source frequency is 50 Hz, since the fluorescent lamp is lit on and off at 100 Hz, the timings of both frequencies coincide at the period of 60 Hz which is three times as high as 20 Hz and at the period of 100 Hz which is five times as high as 20 Hz. Thus, the level fluctuation at the period of 20 Hz occurs in the focal voltage. This directly results in failure of the automatic focusing operation.